1. Field of the Invention
The present invention relates to a ball screw apparatus utilizable suitably for a machine tool, a scalar type robot and other industrial machineries, particular to adjustment of rotational phase shift between a ball screw nut and a ball spline nut.
2. Related Background Art
One of a large variety of known ball screw apparatuses is constructed to move and rotate a single shaft (e.g., Japanese Patent Laid-Open Application No. 61-38892). To be specific, a motor having a hollowed motor shaft is fixed to the inner surface of a housing. A single driving shaft is loosely inserted into the hollowed motor shaft. The driving shaft includes a ball screw groove formed in the rear part of the outer peripheral surface thereof and ball spline grooves formed in the front part thereof, respectively. A ball screw nut is engaged with the ball screw groove in the rear part. A ball spline nut is engaged with the ball spline groove in the front part. At the same time, each nut is revolvably supported through support bearings in the same housing. The ball screw nut is so connected to the rear end of the hollowed motor shaft as to be integrally revolvable. On the other hand, the ball spline nut is disposed opposite to the ball screw nut, with the motor being middled therebetween. A drive converting device consisting of an electromagnetic clutch and a brake is interposed between the motor and the ball spline nut. A rotor of the clutch is fixed to the front end of the hollowed motor shaft. The brake is fixed to the ball spline nut.
Electromagnetic coils of the clutch and the brake are controlled so that when one electromagnetic coil is switched on, the other is switched off. When rotating the motor in a clutch-on/brake-off state, the rotations of the hollowed motor shaft are transmitted to the ball spline nut with rotations of the rotor of the clutch. The ball spline nut revolves because of being in a brake-off state at this moment, whereby the driving shaft rotates. Contrastingly, when rotating the motor in a clutch-off/brake-on state, the rotations of the hollowed motor shaft are not transmitted to the rotor of the clutch. The revolutions of the ball spline nut are hindered by the brake. Hence, the rotations of the hollowed shaft are transmitted to only the ball screw nut, with the result that the driving shaft rectilinearly moves in the axial direction.
The driving shaft can be operated with two degrees of freedom in terms of sliding and rotating by controlling the revolutions of the ball screw nut and the ball spline nut in that manner.
The conventional actuator described above is, however, structured so that the ball screw nut, the motor, the clutch, the brake and the ball spline nut are arranged in line along the single driving shaft in the housing. A whole length of the actuator is therefore too large, and consequently there arises a problem in which the actuator is hard to handle. Additionally, it is required that the motor be demounted if the motor breaks down or when changing the specification. In this case, the following problems are caused. All the parts arranged in series have to be taken out of the housing. The respective nuts attached to the driving shaft through a multiplicity of balls also have to be detached from the driving shaft. It is not easy to replace or change the motor.
Further, in the drive converting device including the electromagnetic clutch and the brake, a slip is easy to cause due to an impact load given from outside. Subtle differences are cumulated because of the repetitive on/off operations. There exists a high possibility to produce a revolving phase shift between the ball screw nut and the ball spline nut. If these conditions are left as they are, there is also caused a problem wherein a positional precision of the driving shaft declines.